The present invention relates to an objective lens for an optical disk used in an optical head for a digital video disk, a digital audio disk, an optical memory disk for a computer, and the like. In particular, the present invention relates to an objective lens for an optical disk including a single lens and capable of satisfying an excellent light focusing property in accordance with any two kinds of optical disks each having a different thickness, an optical head device and an optical information recording and reproducing device using the same, and a mold for molding an objective lens, a method for working the mold and a profile measuring device for measuring the profile of the mold.
In general, an optical head device for an optical disk often employs a single lens having an aspheric surface as an objective lens for recording or reproducing information by focusing light beams into a diffraction limited point image on an information medium. However, in recent years, it is increasingly required to reproduce data compatibly with optical disks each having a different thickness. For example, it is required to read a CD (compact disk) or a CD-ROM having a disk thickness of 1.2 mm and a DVD (digital versatile disk) or DVD-ROM having a disk thickness of 0.6 mm by a single optical head. In this case, one method is to use two objective lenses, one lens corresponding to and one lens corresponding to the CD or CD-ROM having a disk thickness of 1.2 mm and one lens corresponding to the DVD or DVD-ROM having a disk thickness of 0.6 mm. However, from the viewpoint of simplifying an optical system, it is advantageous to use a single lens that is applicable for two kinds of optical disks each having a different thickness. In other words, in the optical head using two objective lenses, since a mechanism for switching lenses is needed, the configuration of the optical head becomes complicated, thus making it difficult to achieve a small size and low cost. Furthermore, it is desirable that the objective lens be as simple as possible.
For example, in JP8 (1996)-334690A and JP9 (1997)-184975A, a so-called two-zone divided type lens is disclosed as an objective lens in order to attain the above-mentioned object. In this configuration, a lens is divided into two regions, in which an inner circumference portion is designed to be suitable for use with a CD or CD-ROM, and an outer circumference region is designed to be suitable for use with a DVD or DVD-ROM. Hereinafter, a conventional objective lens will be described with reference to FIG. 28. FIG. 28 is a view of an arrangement showing a relationship between an objective lens and an optical disk in the prior art.
FIG. 28(a) is a view showing an optical path of a two-sided aspheric objective lens 33 when a light beam is focused onto an optical disk 32 having a thickness of 0.6 mm. The surface of the objective lens at the side of the light source includes an outer circumference region 34 and an inner circumference region 35. In the outer circumference region 34, the spherical aberration is corrected with respect to the optical disk 32 having a thickness of 0.6 mm. On the other hand, in the inner circumference region 35, the spherical aberration is corrected with respect to the optical disk having a thickness of 0.9 mm. The boundary between the inner circumference region 35 and the outer circumference region 34 is determined by the NA that is necessary to reproduce data of the disk having a thickness of 1.2 mm for a light source of a wavelength of 655 nm. For example, when an optical disk having a thickness of 1.2 mm is reproduced at a wavelength of 780 nm and a NA of 0.45, the NA of the light source of 655 nm is about 0.37. The inner circumference region 35 has a spherical aberration with respect to the optical disk having a thickness of 0.6 mm. Herein, the total aberration is much smaller than 0.07xcex (which is called a diffraction limit) and falls in a sufficient aberration for reproducing the optical disk of a thickness of 0.6 mm.
FIG. 28(b) is a view showing an optical path when a light beam is focused onto an optical disk 36 having a thickness of 1.2 mm by using the same objective lens 33. Since the inner circumference region 35 of the same objective lens 33 is optimized with respect to an optical disk having a thickness of 0.9 mm, the aberration is small for the optical disk 36 having thickness of 1.2 mm. However, since the outer circumference region 34 is optimized with respect to an optical disk 32 having a thickness of 0.6 mm, the aberration is large for the optical disk 36 having a thickness of 1.2 mm, so that it does not contribute to the light focusing. Therefore, the outer circumference region 34 also functions as an aperture.
When there is one light source, under the above-mentioned conditions, sufficient performance with respect to two kinds of optical disks each having a thickness of 0.6 mm and 1.2 mm respectively can be attained. However, when it is necessary to reproduce the optical disk having a thickness of 1.2 mm by using the light source of 780 nm like CD-R, the wavelength becomes longer. Consequently, it is necessary to increase the NA relatively, so that the aberration with respect to the optical disk having a thickness of 0.6 mm becomes larger, thus deteriorating the focusing property.
Furthermore, not shown in FIG. 28, a mold for molding an objective lens with a difference in level between an inner circumference portion and an outer circumference portion has been produced by cutting with the use of a diamond cutting tool in order to precisely work a portion having the difference in level.
Furthermore, in a profile measuring device for measuring and evaluating the worked mold, a rotationally symmetric aspheric surface is used as design profile data which is reference data for evaluating measurement values. When a zone-divided type objective lens is measured, the inner circumference portion and the outer circumference portion are measured separately, or a fitting of the profiles of the inner circumference portion and the outer circumference portion is carried out by the use of a higher order aspheric coefficient of about 20-th order to express the profile and the profile is compared with the measurement values.
When the above-mentioned zone-divided type objective lens has the portion having the difference in level between an aspheric surface of the inner circumference portion and an aspheric surface of the outer circumference portion, an ideal design is to provide a surface forming the difference in level approximately parallel to the optical axis. However, in actual working, it is necessary to weaken the surface forming the difference in level to such an extent that the portion having the difference in level can be worked. JP 9 (1997)-184975A discloses the profile in which the portions having the difference in level are smoothly connected. However, it is a profile in which an aspheric surface of the inner circumference portion and an aspheric surface of the outer circumference portion are weakened approximately uniformly. Such a profile can be obtained only by cutting with the use of a cutting tool. Herein, in cutting with the use of the cutting tool, there is a limitation on the type of metal material that can be worked. Unless a relatively soft metal having excellent cutting properties is used, a sufficient working accuracy cannot be obtained. On the other hand, for press molding the lens from a glass material, it is necessary to mold the lens at high temperature and high pressure. It is desirable that a metal material having high hardness, such as a cemented carbide (a sintered metal including WC as a main component) is used. However, the cemented carbide cannot be worked by cutting with the use of cutting tool. In other words, the zone-divided type objective lenses, which have been proposed to date, have a profile for plastic molding. Therefore, most of the conventional objective lenses molded by the use of glass materials have a difficulty in working a mold.
Furthermore, the refractive index of the lens using a plastic material is changed greatly due to a change in temperature. For example, in an optical head device that is necessary to secure operation in a wide range of temperatures, for example, for vehicle apparatus, in general, glass lens is used. However, as mentioned above, a zone divided type DVD/CD compatible lens is designed based on plastic molding. Thus, the temperature characteristics are extremely deteriorated.
Furthermore, in a profile measuring device for measuring and evaluating a mold for molding a lens, the measured profile and the preliminary stored design profile are compared with each other and a working error is calculated. However, a design profile that can be input is only a profile such as a rotationally symmetric aspheric surface. In the method for measuring the zone divided type objective lens by separating the inner circumference portion and the outer circumference portion, since the entire area of the lens is not measured at one time, it is impossible to know the error in profile exactly. Furthermore, in the method of measuring the profile of the entire lens by expressing the profile of the entire lens by fitting with the use of a higher order aspheric coefficient and using this aspheric profile as a design profile, the fitting error occurs around the portion having the difference in level, and thus sufficient measuring accuracy cannot be obtained.
It is an object of the present invention to provide an objective lens for an optical disk including one lens and capable of satisfying an excellent focusing property in accordance with any optical disks each having a different thickness, an optical head apparatus and an optical information recording and reproducing device using the same. It is another object of the present invention to provide a zone-divided type objective lens made of a glass material and excellent in productivity. It is a further object of the present invention to provide a mold for molding the zone-divided type objective lens using cemented carbide capable of being resistant to the molding of glass materials and a method for working the mold. It is a still further object of the present invention to provide a profile measuring device capable of evaluating the profile of the mold for molding the zone-divided objective lens exactly.
In order to achieve the above-mentioned objects, a first configuration of an objective lens for an optical disk according to the present invention includes a single lens having aspheric surfaces on both sides and focusing light beams into a point image through first and second optical disk substrates each having a different thickness, at least one aspheric surface including two regions of an inner circumference region at the inner side of a circular aperture having an optical axis as a center and an outer circumference region located on an outer side with respect to the inner circumference region, the aspheric profile of the outer circumference region correcting a spherical aberration with respect to the first optical disk substrate having a smaller thickness among the first and second optical disk substrates, the aspheric profile of the inner circumference region correcting a spherical aberration with respect to the second optical disk substrate having a larger thickness, and the outer circumference region and the inner circumference region are in contact with each other at the boundary therebetween with a difference in level in the direction of the optical axis, wherein the objective lens satisfies the following relationships (1) to (4);
t1 less than t2xe2x80x83xe2x80x83(1)
0.05 less than TW less than 0.12xe2x80x83xe2x80x83(2)
0.38 less than NA1 less than 0.46xe2x80x83xe2x80x83(3)
0.1 less than p(nxe2x88x921)/xcex21 0.6xe2x80x83xe2x80x83(4)
wherein t1 denotes a thickness of the first optical disk substrate, t2 denotes a thickness of the second optical disk substrate, NA1 denotes a NA of the objective lens at the aperture of the inner circumference region, TPW denotes a wavefront aberration when light beams are focused through the first optical disk substrate (unit is xcex: rms), n denotes a refractive index of the objective lens at the wavelength of a light source when data of the first optical disk are reproduced, p denotes a difference in level between the inner circumference region and the outer circumference region in the direction of the optical axis; and xcex denotes a wavelength of the light source when data of the first optical disk are reproduced.
According to the first configuration of the objective lens for an optical disk, it is possible to obtain an excellent focusing spot with respect to the first optical disk and the second optical disk with one lens. As a result, excellent recording and reproducing characteristics can be obtained.
Furthermore, it is preferable in the first configuration of the objective lens for an optical disk of the present invention that the third order spherical aberration component S3 of the wavefront aberration is approximately 0 when light beams are focused through the first optical disk substrate. According to this preferable configuration, it is possible to minimize the deterioration in performance with respect to an error in the substrate thickness of the optical disk.
Furthermore, it is preferable in the first configuration of the objective lens for an optical disk of the present invention that a fifth order spherical aberration component S5 (unit is xcex: rms) of the wavefront aberration satisfies the following relationship (5) when light beams are focused through the first optical disk substrate.
xe2x88x920.03 less than S5 less than 0.03xe2x80x83xe2x80x83(5)
According to this preferable configuration, it is possible to suppress the light focusing property, in particular, the peak intensity of an Airy ring when the first optical disk is reproduced. Consequently, the deterioration of the reproducing characteristics of the optical disk can be prevented.
Furthermore, it is preferable in the first configuration of the objective lens for an optical disk of the present invention that the thickness t3 of the optical disk substrate satisfies the following relationship (6), when the aspheric profile of the inner circumference region is optimized so that the spherical aberration is corrected with respect to the thickness t3 of the optical disk substrate.
0.8 less than t3 less than 1.2xe2x80x83xe2x80x83(6)
According to this preferable configuration, it is possible to suppress the deterioration factor of the focusing spot for the first optical disk and to reduce the recording density of the second optical disk as compared with the first optical disk so as to reduce the effect of the coma aberration by the tilt of the second optical disk and to suppress the deterioration factor other than the spherical aberration.
Furthermore, it is preferable in the first configuration of the objective lens for an optical disk of the present invention that the cross section of the portion having the difference in level between the inner circumference region and the outer circumference region has a circular arc profile. According to this preferable configuration, working by using a cutting tool, a grindstone, or the like, can be carried out easily.
Furthermore, it is preferable that the first configuration of the objective lens for an optical disk of the present invention is formed by glass molding or plastic molding. According to this preferable configuration, it is possible to cheaply mass-produce lenses having the same profiles and the same performances by previously working a mold of the aspheric profile.
Furthermore, a second configuration of an objective lens for an optical disk according to the present invention includes a single lens having aspheric surfaces on both sides and focusing light beams into a point image through first and second optical disk substrates each having a different thickness, at least one aspheric surface including three regions composed of an inner circumference region at the inner side of a circular aperture having an optical axis as a center, a middle region located at an outer side with respect to the inner circumference region and surrounded by another circular aperture that is located at an outer side of the circular aperture, and an outer circumference region located at an outer side with respect to the middle region, the aspheric profiles of the inner circumference region and the outer circumference region correcting a spherical aberration with respect to the first optical disk substrate having a smaller thickness among the first and second optical disk substrates, and the aspheric profile of the middle region correcting a spherical aberration with respect to an optical disk substrate having a thickness that is larger than the thickness of any of the first and second optical disk substrates, wherein the objective lens for an optical disk satisfies the following relationships (7) and (8);
0.35 less than NA2 less than 0.43xe2x80x83xe2x80x83(7)
0.03 less than NA3xe2x88x92NA2 less than 0.1xe2x80x83xe2x80x83(8)
wherein NA2 denotes a NA of the objective lens at the boundary between the inner circumference region and the middle region, and NA3 denotes a NA of the objective lens at the boundary between the middle region and the outer circumference region.
According to the second configuration of the objective lens for an optical disk, it is possible to obtain an excellent focusing spot with respect to the first optical disk and the second optical disk with one lens. As a result, excellent recording and reproducing characteristics can be obtained.
Furthermore, it is preferable in the second configuration of the objective lens for an optical disk of the present invention that the thickness t4 of the optical disk substrate satisfies the following relationship (9), when the aspheric profile of the middle region is optimized so that the spherical aberration is corrected with respect to the thickness t4 of the optical disk substrate.
1.4 less than t4 less than 2.0xe2x80x83xe2x80x83(9)
According to this preferable configuration, it is possible to correct an aberration with respect to the second optical disk excellently.
Furthermore, it is preferable in the second configuration of the objective lens for an optical disk of the present invention that either the boundary in which the inner circumference region is connected to the middle region or the boundary in which the middle region is connected to the outer circumference region dose not have a difference in level. According to this preferable configuration, it is possible to secure the quantity of light by reducing the ineffective portion occurring at the time of working a lens and to suppress the deterioration of the light focusing property.
Furthermore, it is preferable in the second configuration of the objective lens for an optical disk of the present invention that the cross section of the portion having a difference in level between the inner circumference region and the middle region or the cross section of the portion having a difference in level between the middle region and the outer circumference region has a circular arc profile.
Furthermore, it is preferable in the second configuration of the objective lens for an optical disk of the present invention that the objective lens is formed by glass molding or plastic molding.
Furthermore, a third configuration of an objective lens for an optical disk according to the present invention includes a single lens having aspheric surfaces on both sides and focusing light beams into a point image through first and second optical disk substrates each having a different thickness, at least one aspheric surface including three regions composed of an inner circumference region at the inner side of a circular aperture having an optical axis as a center, a middle region located at an outer side with respect to the inner circumference region and surrounded by another circular aperture that is located at an outer side of the circular aperture, and an outer circumference region located at an outer side with respect to the middle region, the aspheric profiles of the inner circumference region and the outer circumference region correcting a spherical aberration with respect to the first optical disk substrate having a smaller thickness than the second optical disk substrate, the thickness t5 of the optical disk substrate satisfies the following relationship (10) when the aspheric profile of the middle region is optimized so that the spherical aberration is corrected with respect to the thickness t5 of the optical substrate, and the outer circumference region is formed with a difference in level corresponding to an integral multiple of the optical path length in the direction of an axis with respect to the inner circumference region, wherein the objective lens satisfies the following relationships (11) to (13).
1.0 less than t5 less than 1.4xe2x80x83xe2x80x83(10)
t1 less than t2xe2x80x83xe2x80x83(11)
0.35 less than NA2 less than 0.43xe2x80x83xe2x80x83(12)
0.03 less than NA3xe2x88x92NA2 less than 0.1xe2x80x83xe2x80x83(13)
wherein t1 denotes a thickness of the first optical disk substrate, t2 denotes a thickness of the second optical disk substrate, NA2 denotes a NA of the objective lens at the boundary between the inner circumference region and the middle region, and NA3 denotes a NA of the objective lens at the boundary between the middle region and the outer circumference region.
According to the third configuration of the objective lens for an optical disk, it is possible to obtain an excellent focusing spot with respect to the first optical disk and the second optical lens with one disk. As a result, excellent recording and reproducing characteristics can be obtained.
Furthermore, it is preferable in the third configuration of the objective lens for an optical disk of the present invention that the focal position in which a wavefront aberration of the inner circumference region is minimized and the focal position in which a wavefront aberration of the outer circumference region is minimized are the same when light beams are focused through the second optical disk substrate. According to this preferable configuration, when light beams reflected from an information medium surface are incident in a photodetector, they return to the same portions with respect to the inner circumference region and the outer circumference region. As a result, it is possible to obtain an accurate signal light.
Furthermore, it is preferable in the third configuration of the objective lens for an optical disk of the present invention that the third order spherical aberration component S3 of the wavefront aberration in the ranges of the inner circumference region and the middle region is approximately 0 when light beams are focused through the second optical disk substrate. According to this preferable configuration, it is possible to record and reproduce the second optical disk excellently.
Furthermore, it is preferable in the third configuration of the objective lens for an optical disk of the present invention that the boundary in which the inner circumference region is connected to the middle region does not have a difference in level.
Furthermore, it is preferable in the third configuration of the objective lens for an optical disk of the present invention that the boundary between the middle region and the outer circumference region is provided at the intersection point of the profiles of the middle region and the outer circumference region. Furthermore, it is preferable in the third configuration of the objective lens for an optical disk of the present invention that both the boundary in which the inner circumference region is connected to the middle region and the boundary in which the middle region is connected to the outer circumference region do not have a difference in level. According to such a preferable configuration in which the boundary between the middle region and the outer circumference region is provided at the intersection point of the profiles of the middle region is connected to the outer circumference region and both the boundary in which the inner circumference region is connected to the middle region and the boundary in which the middle region is connected to the outer circumference region do not have a difference in level, it is possible to facilitate the work by removing a portion having the difference in level from the aspheric profile and to suppress the loss of the quantity of light by removing the ineffective region from the surface profile.
Furthermore, it is preferable in the third configuration of the objective lens for an optical disk of the present invention that the objective lens is produced by glass molding or plastic molding.
As mentioned above, in the first to third configurations of the objective lens for an optical disk of the present invention, the aberration correction of a single lens is designed so as to obtain a focusing performance necessary to the substrate thickness of two optical disks, respectively. In the aberration of the lens having a smaller thickness of the optical disk substrate and a higher NA, since it is necessary to take the aberration at the optical disk having a large thickness into account, the total aberration becomes larger than is conventional. However, in this aberration, by designing the different profiles in the outer circumference region and the inner circumference region, and at the same time by providing the boundary between the outer circumference region and the inner circumference region with the surface forming the difference in level in the direction of the optical axis, it is possible to maintain the profile of the spot at the side of the optical disk in which information is recorded with a high density. Furthermore, in the aberration of the lens having a larger thickness of the optical disk substrate and lower recording density and lower NA, by suppressing the aberration in the necessary aperture sufficiently and deteriorating the aberration rapidly at the outside of the necessary aperture, it is possible to provide the same effect as having a diaphragm. As a result, it is possible to record information or reproduce information with a stable performance.
Furthermore, a first configuration of an optical head device according to the present invention includes two light sources, a focusing means for focusing light beams emitted from the two light sources onto the information medium surface through first and second optical disk substrates each having a thickness corresponding to the respective light source, a light flux separating means for separating light fluxes modulated at the information medium, and a light receiving means for receiving the light beams modulated at the information medium, wherein the focusing means is one of the first to third configurations of the objective lens for an optical disk according to the present invention.
Furthermore, a configuration of an optical information recording and reproducing apparatus according to the present invention, recording information on the information medium of surfaces of the first and second optical disk substrates each having a different thickness, or reproducing information recorded on the information medium surfaces by the use of an optical head deice, wherein the first configuration of the optical head device according to the present invention is used as the optical head device.
According to the first configuration of the optical head device and the configuration of the optical information recording and reproducing apparatus, since it is possible to record and reproduce information with one objective lens with respect to two kinds of optical disks each having a different substrate thickness, a cheap optical head device and optical information recording and reproducing apparatus can be realized. Furthermore, by providing any of the optical disks with aberration suitable to each disk in accordance with the aperture of the objective lens, it is possible to obtain an excellent recording, reproducing and erasing performance with respect to the two kinds of optical disks with one objective lens.
Furthermore, a fourth configuration of an objective lens for an optical disk according to the present invention includes a single lens made of glass and which focuses light beams into a point image through first and second optical disk substrates each having a different thickness, at least one surface is divided into at least three regions by concentric circles having an optical axis as a center, and among the three regions, a first region including the optical axis and a second region located at an outermost part are rotationally symmetric aspheric surfaces, and a third region sandwiched by the first region and the second region is a toric surface having the optical axis as an axis of rotation. According to the fourth configuration of the objective lens for an optical disk, various advantageous are provided as compared with a conventional two-zone divided type objective lens having a sharp difference in level. In other words, since the conventional objective lens has a sharp difference in level, it is required that the mold must be worked by the use of a diamond cutting tool. As a result, material to be molded is limited. However, according to the fourth configuration of the objective lens for an optical disk, since a portion having a difference in level is formed by the toric surface having the optical axis as an axis of rotation, it is possible to work the mold with a grindstone. Thus, it is possible to produce a lens by using a glass material to be molded at high temperature and high pressure.
Furthermore, it is preferable in the fourth configuration of the objective lens for an optical disk of the present invention that the third region that is the toric surface is in contact with the second region and intersects the first region.
Furthermore, it is preferable in the fourth configuration of the objective lens for an optical disk the present invention that aspheric coefficients of the first region and the second region are different from each other.
Furthermore, it is preferable in the fourth configuration of the objective lens for an optical disk of the present invention that the toric surface is provided on the surface having a larger central radius of curvature of the lens.
Furthermore, it is preferable in the fourth configuration of the objective lens for an optical disk of the present invention that the third region is a toric surface obtained by rotating a circular arc with an optical axis as a center, the circular arc having a radius R satisfying the following relationship (14).
0.7 mm less than R less than 2.5 mmxe2x80x83xe2x80x83(14)
When R is 0.7 mm or less, a mold cannot be worked by grinding work, thus making it difficult to produce the lens. On the other hand, when R is 2.5 mm or more, the width of the arc circular portion is too broad, thus deteriorating the performance of the lens.
Furthermore, it is preferable in the fourth configuration of the objective lens for an optical disk of the present invention that the third region is a toric surface obtained by rotating a circular arc with an optical axis as a center, the circular arc having a radius R satisfying the following relationship (15).
1.6 mm less than R less than 2.1 mmxe2x80x83xe2x80x83(15)
Furthermore, it is preferable in the fourth configuration of the objective lens for an optical disk of the present invention that the width w of the third region satisfies the following relationship (16).
0.02 mm less than w less than 0.04 mmxe2x80x83xe2x80x83(16)
When w is 0.02 mm or less, the lens has a profile that cannot be worked by using a grindstone, thus making it difficult to produce the lens. On the other hand, when w is 0.04 mm or more, the difference in profile between an actually obtained lens and an ideally designed lens is too large, and thus sufficient lens performance cannot be exhibited.
Furthermore, a second configuration of the optical head device according to the present invention includes a first light source emitting light beams having a first wavelength, a second light source emitting light beams having a second wavelength different from the first wavelength, a focusing means for focusing light beams from the first and second light sources onto an information recording medium, a first light receiving means for receiving the light beams having the first wavelength among the reflecting light beams from the information recording medium and a second light receiving means for receiving the light beams having the second wavelength, wherein the focusing means is the fourth embodiment of the objective lens for an optical disk according to the present invention.
According to the second configuration of the optical head device, since the fourth configuration of the optical lens for an optical disk of the present invention is used, it has the following advantageous over the optical head device using the conventional zone-divided type objective lens. Namely, since the fourth embodiment of the objective lens for an optical disk of the present invention is an objective lens made of a glass material having less deterioration in the aberration due to a change in temperature, for example, it is possible to realize an optical head device necessary to secure an operation under a wide range of temperature conditions, for example, for a vehicle apparatus, etc. Furthermore, as mentioned above, the production cost of the mold can be reduced as compared with a conventional objective lens, and furthermore, a longer lifetime of the mold is expected to be attained. Consequently, it is possible to reduce the production cost of an objective lens and in turn the production cost of the optical head device.
Furthermore, a configuration of a mold for molding a lens according to the present invention is as follows. The mold is produced by grinding cemented carbide. A lens molding surface is divided into at least three regions by concentric circles having an optical axis as a center, and among the three regions, a first region including the optical axis and a second region located at an outermost part are rotationally symmetric aspheric surfaces, the third region which is sandwiched by the first region and the second region is a toric surface having the optical axis as an axis of rotation. According to the configuration of the mold for molding the lens, various advantages are obtained over the conventional two-zone divided type objective lens, because a portion having a difference in level is formed of the toric surface having the optical axis as an axis of rotation. Namely, production by using the grindstone is possible and a super-hard material suitable for molding a glass material can be used as a material for the mold. Furthermore, since there is no roughness having a small vertex radius of curvature within an effective diameter, it is possible to carry out profile measuring at high accuracy by the use of a probe having a tip radius of curvature of about 500 xcexcm.
Furthermore, it is preferable in the configuration of the mold for molding the lens of the present invention that the radius of curvature R of the toric surface having the optical axis as an axis of rotation satisfies the following relationship (17).
0.7 mm less than R less than 2.5 mmxe2x80x83xe2x80x83(17)
Furthermore, it is preferable in the configuration of the mold for molding the lens of the present invention that the radius of curvature R of the toric surface having the optical axis as an axis of rotation satisfies the following relationship (18).
xe2x80x831.6 mm less than R less than 2.1 mmxe2x80x83xe2x80x83(18)
If a grindstone having a radius that is the same as or smaller than the above-mentioned radius of curvature R is used, grinding work can be possible. Therefore, a cemented carbide (for example, a sintered metal including WC as a main component) capable of molding a high melting glass material under high pressure can be used as a material for the mold.
Furthermore, a method for working a mold for molding a lens according to the present invention includes grinding a lens molding surface of the mold for molding the lens by the use of a diamond grindstone. The lens molding surface is divided into at least three regions by concentric circles having an optical axis as a center, and among the three regions, a first region including the optical axis and a second region located at an outermost part being rotationally symmetric aspheric surfaces, and the third region which is sandwiched by the first region and the second region being a toric surface having the optical axis as an axis of rotation. A radius of the diamond grindstone used for the grinding work is the same as or smaller than the radius of curvature of the toric surface. According to the method for working a mold for molding a lens, since the method includes grinding work using a diamond grindstone, it is possible to produce a two-zone divided type mold for molding a lens which is made of a cemented carbide.
Furthermore, it is preferable in the method for working a mold for molding a lens of the present invention that the radius R of the diamond grindstone satisfies the following relationship (19).
0.7 mm less than R less than 2.5 mmxe2x80x83xe2x80x83(19)
When R is 0.7 mm or less, the diameter of a grindstone is too thin, and the accuracy of the worked surface becomes bad. On the other hand, when R is 2.5 mm or more, the toric surface region (connecting region) is broadened, and the aberration of the molded lens is deviated greatly from the design value.
Furthermore, it is preferable in the method for working a mold for molding a lens of the present invention that the radius R of the diamond grindstone satisfies the following relationship (20):
1.6 mm less than R less than 2.1 mmxe2x80x83xe2x80x83(20)
When the radius R of the grindstone is within this range, it is possible to reduce the width of the toric surface region and at the same time to maintain the diameter of the grindstone so as to have a sufficient intensity.
Furthermore, a configuration of a profile measuring device according to the present invention includes a precision stage, a controller for controlling the precision stage, a length measuring means, a means for inputting design profile data, and a means for outputting a difference between the design profile data and measured data; wherein as the design profile data, a first rotationally symmetric aspheric surface is used in the region where the distant from the optical axis is less than the radius h1, a second rotationally symmetric aspheric surface is used in the region where the distance from the optical axis is not less than the radius h2, and a toric surface having the optical axis as a rotationally symmetric axis is used in the region between the radius h1 and the radius h2. According to the configuration of the profile measuring device, it is possible to express one surface profile by using three surfaces including an inner aspheric surface, an outer aspheric surface and a toric surface having the optical axis of the middle region between the inner aspheric surface and outer aspheric surface as an axis of rotation. Therefore, the profile measuring device of the present invention has various advantages over the conventional profile evaluating device. In other words, when the fitting of the surface profile is carried out by the use of a higher order multinomial expression, fitting error surely occurs. In particular, when there is a portion having a difference in level, the fitting results are varied around the portion of the difference in level and the error becomes larger, and thus the measuring accuracy is deteriorated. However, according to the configuration of the profile measuring device of the present invention, it is possible to measure the profile faithfully based on the design profile, and thus the measuring accuracy can be improved greatly.